1. Field of the Invention
The present invention relates to a color filter, a method of fabricating the same and a liquid crystal display (LCD) panel including the same.
2. Description of Related Art
Liquid crystal displays (LCD) become the mainstream in recent years due to the slim shape, light weight, low operation voltage, low power consumption and radiation-free operation. However, there are still a number of technical limitations for LCDs. Therefore, most of the LCD products are small-dimension display devices.
For large-dimension display devices, those using the projection technology, such as reflective projection display devices and rear projection display devices, are in the development. The reflective projection display devices include liquid crystal projectors (LCP), digital light processing (DLP) projectors and liquid crystal on silicon (LCOS) projection display devices. The LCOS projection technology can also be applied to the rear projection display devices. Although the current market focuses on the liquid crystal projectors and digital light processing projectors, many manufacturers have developed the LCOS projection technology due to the low cost, high aperture ratio (up to 90%) and high resolution (e.g., pixel pitch=12 μm or less).
For a display device, a color filter is required to filter the lights emitted from a light source. A typical color filter is usually formed by interlacing and stacking several color filter layers with different refraction indices so that certain wavelengths are filtered. One of known methods for fabricating a color filter is by performing a lift-off process. First, a photoresist layer shaped as inverted trapezoids between openings in a vertical cross section is formed over a substrate. Thereafter, color filter layers with different refraction indices are sequentially formed on the substrate covering the photoresist layer and filling the openings therein. Since the sidewalls of the inverted trapezoids are difficult to be covered by the color filter layers when the angle thereof gets larger, the color filter layers on the photoresist layer can be removed by using a lift-off process so that the color filter layers on the substrate remain. A negative photoresist is frequently used for forming the inverted trapezoid photoresist layer, and the sidewall angle is affected by the thickness of the photoresist layer and the reaction of the photoresist agents. In the fabricating process, the sidewall angle is controlled by the exposure dose and post-exposure baking temperature; thus, the sidewall angle is confined and large-angled inverted trapezoids are impossible.
Another method for fabricating a color filter is by performing a lift-off process and an etching process at the same time. A first color filter layer is formed on a substrate. An etching process is then performed, using a patterned photoresist layer as a mask, to the first color filter layer so as to form a first color filter pattern. Thereafter, a second color filter layer is formed over the substrate covering the patterned photoresist layer and a portion of the substrate that is exposed by the first color filter pattern. Afterwards, a lift-off process is performed to simultaneously remove the patterned photoresist layer and the second color filter layer thereon, so that the second color filter layer in the openings of the first color filter pattern remains to form a second color filter pattern. A gap fill layer is then formed.
However, a serious optical problem occurs in the above-mentioned method when color filter layers are formed by depositing and interlacing inorganic films. In detail, because the patterned photoresist layer and the first color filter pattern are very thick, a shadow effect is caused in deposition of the second color filter layer; thus, a gap exists between the second color filter layer and the adjacent first color filter pattern resulting in a lower aperture ratio. Also, the thickness of the second color filter layer in the openings of the first color filter pattern is not uniform due to the shadow effect, and the thickness at the center is larger than that in the periphery. The smaller thickness in the periphery causes certain problems to the optical performance, such as a reduced reflection area, an altered refraction index and a shifted spectrum. Further, the subsequently formed gap fill layer is difficult to fill in due to the non-uniform thickness of the second color filter layer and a rough topography is caused; thus, an alignment problem occurs in the later formed liquid crystal layer between the two substrates. Moreover, since the photoresist layer is removed by the lift-off process, the cleaning degree of the process is difficult to control.